Antenna design for surface wave suppression

ABSTRACT

Means for suppressing the excitation of electromagnetic surface waves on a dielectric converter antenna. A hole is cut in the dielectric where it covers the antenna. A plug of the dielectric material is coated around its rim with a metal and then fitted into the hole such that the metal coating extends the conducting surface of the antenna through the dielectric material. The metallic coating minimizes the possibility of surface wave excitation by confining the waves reflected by the dielectric-air interface to the chamber formed by the coating.

William L. Grantham Yorktown, Va.

Dec. 23, 1969 May 18, 197 l the United States of America as representedby the Administrator of the National Aeronautics and SpaceAdministration Inventor Appl. No. Filed Patented Assignee ANTENNA DESIGNFOR SURFACE WAVE SUPPRESSION 6 Claims, 1 Drawing Fig.

U.S. Cl 343/708, 343/784, 343/872 Int. Cl H0lq 1/28, i-lOlq 13/00 Fieldof Search 343/705,

[56] References Cited UNITED STATES PATENTS 3,346,865 10/1967 Jones343/708 3,384,895 5/1968 Webb 343/708 Primary Examiner-Eli LiebermanAtt0meysWilliam H. King, Howard J. Osborn and G. T.

McCoy Patentd May 18, 1971 3,579,242

INVENTOR. WILLIAM L. GRANTHAM BY QAQ ATTOR ORIGIN OF THE DISCLOSURE Theinvention described herein was made by an employee of the United StatesGovernment and may be manufactured and used by or for the Government forgovernmental purposes without the payment of any royalties thereon ortherefore.

BACKGROUND OF THE INVENTION The invention relates generally to antennasand more specifically concerns a spacecraft antenna design for surfacewave suppression.

A spacecraft's outside surface is usually covered with a layer ofdielectric material for heat protection to the spacecraft. Duringhypervelocity entry of the spacecraft into the earth's atmosphere thelayer of dielectric material ablates thereby keeping the temperature ofthe spacecraft below a critical value. Each existing antenna which mustbe covered by this layer of dielectric material, permits the excitationof electromagnetic surface waves which seriously degrade the antenna'spattern and the antennas usefulness as a plasma diagnostic sensor. It istherefore the primary purpose of this invention to provide means forsuppressing the excitation of electromagnetic surface waves ondielectric covered antennas.

SUMMARY OF THE INVENTION The invention is applicable to horn, T-fed slotand similarly constructed antennas. It consists of a thin metallicwalled extension of the antennas conducting surfaces through the layerof dielectric material that covers the spacecraft. The use of themetallic walled extension minimizes the possibility of surface waveexcitation by confining the waves reflected by the dielectric-airinterface to a chamber formed by the metallic walls. The reflected waveswould normally contribute appreciable to the generation of surface wavesand thus to the degradation of antenna performance. The thin metallicwall, about i to microns thick, is deposited by some suitable technique,such as vapor depositing, on a dielectric plug the same size as theantenna aperture and inserted in a hole provided for it at the apertureof the antenna. The main feature of the device that constitutes thisinvention is its ability to provide the electrical improvement desiredand, by the use of thin walls, not impair the ablative character of thedielectric material. The thin metallic walls recess with the dielectricas ablation occurs so that surface protrusions and recessions whichwould seriously affect the dielectric's heat protection capability areavoided.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE in the drawing is aschematic diagram of the invention.

DETAILED DESCRIPTION OF THE INVENTION Turning now to the embodiment ofthe invention selected for illustration in the drawing the number 11designates a cross section of the outside structure of a spacecrafthaving an outside surface 12. A horn antenna 13 is mounted inside thespacecraft such that it extends through outside structure 11 and isflush with outside surface 12. Even though a horn antenna is disclosedit is to be understood that the invention is also applicable to T-fedslot and similarly constructed antennas. Outside surface 12 and theaperture of antenna 13 are covered with a layer of dielectric material14. The purpose of this layer of dielectric material is to provide heatprotection for the spacecraft as it enters the earths atmosphere at ahypervelocity. As the spacecraft enters the earth's atmosphere thedielectric material 14 ablates thereby maintaining the temperature ofthe spacecraft below a critical value. Ordinarily electromagnetic wavesemitted from antenna 14 are partially reflected by the dielectric-airinterface at the forward surface of layer 14. These reflected wavescontribute appreciably to the generation of surface waves in thedielectric and thus to the degradation of antenna performance.

The present invention consists of cutting a hole in layer 14 over theaperture of antenna 13 that is slightly larger than the aperture of theantenna. A plug 15 of the dielectric material is coated on its rim by ametal to form a thin layer 16 of the metal all along the rim. The metalcan be the same metal as in antenna 13 or any electrically conductingmetal. The metal layer 16 is formed on the rim of plug 15 by anysuitable technique such as vapor deposition and is about 1 to .10microns thick. Plug 15 with layer 16 is fitted into the hole in thelayer 14 of dielectric material so that metallic layer 16 makes contactwith antenna 13 around the periphery of the aperture of the antenna. Theuse of the metallic wall formed by layer 16 minimizes the possibility ofsurface wave excitation by confining the waves reflected by thedielectric-air interface to a chamber formed by the metallic wall.

The advantages of this invention are its ability to provide theelectrical improvement required and not impair the ablative character ofthe dielectric material. The thin metallic layer 16 recesses with thedielectric material 14 as ablation occurs so that surface protrusionsand recessions are avoided which would seriously affect the dielectricsheat protection capability.

What is claimed as new and desired to be secured by Letter Patent of theUnited States is:

Iclaim:

1. Apparatus comprising:

an antenna mounted on a spacecraft such that the antenna is flush withthe outside surface of the spacecraft;

a heat protection layer of dielectric material extending over theoutside surface of the spacecraft and covering said antenna;

a thin metallic walled extension of said antenna's conducting surfacethrough said layer of dielectric material whereby electromagneticsurface waves are suppressed by the metallic wall and the metallic walldoes not impair the ablative character of the dielectric material.

2. Apparatus according to claim 1 wherein said thin metallic wallcomprises a dielectric plug the same size as the antenna aperture coatedwith a thin metallic wall and inserted into the hole provided for it atthe antenna.

3. Apparatus according to claim 2 wherein said antenna is a hornantenna.

4. Apparatus according to claim 2 wherein said coating is about 1 to l0microns thick.

5. Method of making apparatus for suppressing the excitation ofelectromagnetic surface waves on an antenna covered by a layer ofdielectric material comprising the steps of:

cutting a hole in said layer of dielectric material over the aperture ofsaid antenna;

cutting a plug of said material slightly smaller than said hole;

coating the rim of said plug with a thin layer of metal so that thecoated plug is the same size as the hole; and

fitting said coated plug into said hole so that said layer of metalmakes contact with said antenna.

6. Apparatus for suppressing the excitation of electromagnetic surfacewaves on an antenna covered by a layer of dielectric materialcomprising:

a hole in said layer of dielectric material over the aperture of saidantenna; and

a plug of said dielectric material, coated on its rim with a thin layerof metal, fitted into said hole such that said layer of metal makescontact with said antenna.

1. Apparatus comprising: an antenna mounted on a spacecraft such thatthe antenna is flush with the outside surface of the spacecraft; a heatprotection layer of dielectric material extending over the outsidesurface of the spacecraft and covering said antenna; a thin metallicwalled extension of said antenna''s conducting surface through saidlayer of dielectric material whereby electromagnetic surface waves aresuppressed by the metallic wall and the metallic wall does not impairthe ablative character of the dielectric material.
 2. Apparatusaccording to claim 1 wherein said thin metallic wall comprises adielectric plug the same size as the antenna aperture coated with a thinmetallic wall and inserted into the hole provided for it at the antenna.3. Apparatus according to claim 2 wherein said antenna is a hornantenna.
 4. Apparatus according to claim 2 wherein said coating is about1 to 10 microns thick.
 5. Method of making apparatus for suppressing theexcitation of electromagnetic surface waves on an antenna covered by alayer of dielectric material comprising the steps of: cutting a hole insaid layer of dielectric material over the aperture of said antenna;cutting a plug of said material slightly smaller than said hole; coatingthe rim of said plug with a thin layer of metal so that the coated plugis the same size as the hole; and fitting said coated plug into saidhole so that said layer of metal makes contact with said antenna. 6.Apparatus for suppressing the excitation of electromagnetic surfacewaves on an antenna covered by a layer of dielectric materialcomprising: a hole in said layer of dielectric material over theaperture of said antenna; and a plug of said dielectric material, coatedon its rim with a thin layer of metal, fitted into said hole such thatsaid layer of metal makes contact with said antenna.